Quantitative Analysis: Acid-Base Titrations

What a titration determines

A solution may be described as quantitative when measurements are used to find a numerical amount. In an acid–alkali titration, a measured volume of one solution reacts with a carefully measured volume of another. If one concentration is known, the reacting volumes and the balanced equation can be used to determine the unknown concentration.

OCR requires learners to be able to determine the concentrations of strong acids and strong alkalis. Titration is an appropriate way to do this because strong acids and strong alkalis neutralise one another, and a suitable indicator can reveal when just enough solution has been added.

For example, hydrochloric acid and sodium hydroxide react as follows:

$\mathrm{HCl + NaOH \rightarrow NaCl + H_2O}$

The coefficients show that one mole of HCl reacts with one mole of NaOH. This mole ratio, rather than the measured volumes alone, connects the titration result to concentration.

The apparatus and what each item does

A typical accurate titration uses:

• a volumetric pipette and pipette filler to transfer one fixed, accurate volume of solution;
• a burette, held vertically by a clamp and stand, to measure a variable volume accurately;
• a conical flask, whose shape allows the mixture to be swirled without spilling;
• a small funnel for filling the burette, removed before readings and delivery;
• a suitable indicator, added in only a few drops;
• a white tile, which makes a faint colour change easier to see.

Never pipette by mouth: use a pipette filler. Wear eye protection, handle acids, alkalis and fragile glassware carefully, and fill the burette below eye level to reduce the danger from splashes.

Measuring the reacting volumes

Use the pipette and filler to transfer a fixed volume of the solution being investigated into the conical flask. Add a few drops of a suitable indicator and place the flask on a white tile.

Clamp the burette vertically, close its tap and fill it with the other solution. Run some liquid through the tap so that the jet is full and contains no air bubble. Remove the filling funnel, then read and record the initial burette reading. View the bottom of the meniscus at eye level so that the line of sight does not introduce parallax error.

Carry out a rough titration first. Add solution from the burette while swirling the flask so that the reactants mix. The rough result locates the end-point but is not included in the final mean.

Repeat with fresh solution in the flask. Add the burette solution fairly quickly at first, then drop by drop as the rough titre is approached. Keep swirling. Stop at the first required permanent colour change and record the final burette reading.

The delivered volume is

$\text{titre}=\text{final burette reading}-\text{initial burette reading}$

Repeat accurate titrations until concordant titres are obtained. For the method represented by the supplied practical evidence, this means results within $0.10\ \text{cm}^3$ of each other. Calculate the mean from concordant accurate titres only, excluding the rough result and any anomalous titre.

Why these details improve accuracy

Adding liquid dropwise near the end-point matters because one extra drop can take the mixture beyond the colour change and make the titre too large. Constant swirling ensures that each addition reacts throughout the solution rather than creating a temporarily acidic or alkaline region near the burette tip. A white tile makes the first faint change easier to judge consistently.

The end-point is the observed indicator colour change. It is used as an experimental sign that the acid and alkali have reacted in the required proportions. Only a few drops of indicator are used so that the indicator itself has a negligible effect on the reacting volumes.

A volumetric pipette is preferred to a measuring cylinder for the fixed volume because it is designed to deliver one volume accurately. A burette is used for the added solution because it measures a range of delivered volumes accurately rather than only one fixed volume.

Turning a titre into concentration

The central relationship is

$n=cV$

where $n$ is amount in moles, $c$ is concentration in $\text{mol dm}^{-3}$ and $V$ is volume in .

Laboratory volumes are normally recorded in $\text{cm}^3$, so convert them before using this equation:

$V\text{ in dm}^3=\frac{V\text{ in cm}^3}{1000}$

Use this sequence:

1. Calculate the mean concordant titre.
2. Convert the mean titre from $\text{cm}^3$ to $\text{dm}^3$.
3. Use $n=cV$ to calculate the moles of the solution whose concentration is known.

$c=\frac{n}{V}$

For HCl reacting with NaOH, the ratio is $1:1$, so the reacting moles are equal at the end-point. This shortcut cannot be transferred automatically to every acid–alkali pair. For example, the balanced equation must be consulted whenever one formula unit can supply or react with more than one hydrogen ion or hydroxide ion.

Judging the quality of the result

Repeated concordant titres show precision: the method gives closely grouped values. They do not, by themselves, prove that the result is accurate. A consistent procedural error—such as repeatedly reading the meniscus from above, leaving an air bubble in the burette tip or overshooting the colour change—could shift every titre in the same direction.

A strong evaluation therefore uses both the spread of the repeats and the quality of the technique. State the likely effect where possible: overshooting the end-point makes the measured titre too large, while parallax can make a reading either too high or too low depending on the viewing position.